In the recent years, a variety of widely spread wireless communication networks have been developed. These networks use different access technologies and range from wireless personal area networks to satellite based networks globally available. Between the above mentioned extremes, there are a variety of different networks and network technologies available that provide local, regional or country wide coverage, such as Bluetooth, Wireless-LAN using the IEEE 802.11 standard or cellular telecommunication networks such as GSM or UMTS.
Upon movement of a user, different network access technologies may be available to the user terminal. Small areas with a typical size of a building are often covered by networks using Bluetooth, DECT or wireless-LAN (W-LAN) technologies. In a private environment the user e.g. favors using a headset or the like supporting Bluetooth technology or using the service provided by a W-LAN hotspot, that is typically provided at airports or railway stations, internet cafes, etc.
In an urban environment the user terminal can be connected to a cellular network, whilst upon movement to a more rural area the service may be provided by a different network provider. So the network technology e.g. GSM or UMTS may remain unchanged but the access network and the service provider changes. Upon further location change, e.g. when the user crosses the borders of a country, the need for roaming between networks of different providers and even handovers between different network technologies very likely arises.
Or the user may move to areas where no network service is available except for satellite based network service. Also in this situation there may be the necessity to handoff the mobile terminal e.g. from a UMTS network to a satellite based network.
But not only the handoff of mobile terminals between different access networks and different access technologies, also parallel communication with different networks, network technologies or different access points may be of interest. Upon movement of a user to an airport or a railway station, it may be of interest to receive services e.g. an updated timetable via a local wireless LAN hotspot service (W-LAN), while remaining connected to e.g. a voice service provided by UMTS.
In summary, nowadays a user terminal upon movement may utilize a variety of different wireless communication networks, network technologies and services that are available.
Not only is it advantageous for the customer, it may also be desirable for the network service provider to handoff a mobile terminal from one network to another. For instance, it might be of interest to handoff the mobile terminal from a cellular network to a neighboring cellular network or in case the user enters a W-LAN hotspot zone to perform a handoff between the cellular network and the W-LAN network.
Different networks and network technologies can be tightly coupled with the aid of the Multi-radio Unification Protocol (“A Multi-Radio Unification Protocol for IEEE 802.11 Wireless Networks”, Adya et. al, Proceedings of the first international conference on broadband networks, BROADNETS'04). This protocol is applicable to wireless communication networks such as IEEE 802.11 or UMTS. In such a heterogeneous network it is not only desired to handoff the mobile terminal easily across the various networks; the mobile terminal may also be capable of communicating with more than one radio access point at the same time, using the respective radio access technique the access point supports.
The option of parallel data transmission leads inter alia to an increase of reliability of the transmission and to higher data throughput. A further advantage of parallel transmission is the optimized use of different bandwidths offered by the respective access point, i.e. the respective radio access technique. Data related to different applications such as voice service or data download may be provided to the mobile terminal using a radio access technique offering the appropriate bandwidth, e.g. IEEE802.11 may be used for high speed data packet services and UMTS may be used for the voice services.
It may be even possible within one application that e.g. the audio stream is provided by the radio access technique offering the lower bandwidth and the video stream is provided by the radio access technique offering the higher bandwidth. Additional to bandwidth also the data delay is desired to be in a certain range appropriate to the specific application. Some applications e.g. data download are delay insensitive, other like Voice-over-IP (VoIP) or video streaming are not.
In radio access networks, transmission of the same data flow via different radio access points is possible at different transmission time intervals (TTI). This possibility implies the switching of the radio access point and radio access technique respectively, through which data transmission is done. This radio access switching may be in certain cases very dynamic, e.g., sometimes it can be done at a TTI level.
In order to provide best quality of service to the mobile terminal in these radio access networks a multi-radio Automatic Repeat Request (ARQ) mechanism is typically implemented. The term multi-radio ARQ means that link layer acknowledgements for data packets can be transmitted via a different radio access point than the one used for the data transmission. The motivation for introducing multi-radio ARQ is to avoid cases, where the radio access point switching periods may be very long. This might happen, in some cases when data and acknowledgement transmission is done via the same radio access point, as the delay on uplink and downlink may be significantly different.
To allow very dynamic radio access point switching in presence of delay intolerant services provided to the mobile terminal (e.g. Voice-over-IP or video streaming), it is desirable that acknowledgement messages for data packets are transmitted without considerable delays or at least with delays that are not higher than the ones observed in the case of absence of multi-radio ARQ mechanisms. Further, in order to prevent abnormal situations in the sense that e.g. data streaming stagnates or needless retransmissions of data packets occur, due to missing acknowledgement messages it is desirable to send acknowledgement messages as fast as possible.